Evaluation of Cocoa Types for Resistance to Capsids

Abstract

Ten clones and 10 hybrids each of four cocoa populations; Nanays (Na), IMCs, Trinidad introduction (Ts) and Parinaris (Pa), were screened for attractiveness and resistance/tolerance to the capsid, Sahlbergella singularis (Hagl.). Two screening methods; a laboratory ‘microtest’ and an insectary test were used to determine the attractiveness of the genotypes to capsids. In addition, seedlings exposed to capsids in the insectary were observed under ‘ semi-field’ conditions to assess their tolerance and ability to outgrow capsid damage. The laboratory microtest on the clonal materials established the Parinaris (2.27 lesions/chupon) as the most preferred population followed by the Ts (2.20 lesions/ chupon), the IMCs (1.97 lesions/chupon) and the Nanays (1.55 lesions/chupons) in that order. Pa 7/808 (3.25 lesions/chupon) was the most susceptible clone. The Nanays, Na 744 (1.15 lesions/chupon), Na 225 (1.19 lesions/chupon), Na 427 (1.29 lesions/ chupon) and Na260 (1.38 lesions/chupon) were the least susceptible clones. Results from the insectary preference tests followed the same order with the Parinari clones emerging as the most preferred and the Nanays as the least preferred. Preference test on hybrids in the insectary showed that the IMC crosses (4.99 lesions/seedling) were the most preferred, followed by the Parinari crosses (4.78 lesions/seedling), the T crosses (4.71 lesions/seedling) and Nanays (4.18 lesions/ seedling) in that order. In the semi-field tests, some level of tolerance was observed in both the clonal and hybrid materials but growth measurements showed that the latter were more vigorous. The Parinari clones recorded the highest mean increase in height (25.8cm in 8 weeks) whilst the remaining three populations, IMCs (9.5cm in 8 weeks), Ts (8.7cm in 8 weeks) and Na (5.9cm in 8 weeks) showed no significant differences (P>0.05) in growth. For the hybrids, the Nanays (32.0cm in 8 weeks), Ts (28.9cm in 8 weeks) and the Parinaris (27.65cm in 8 weeks) showed no significant differences in increments in height (P>0.05) but growth in all three populations were significantly higher than in the IMC population (19.24cm in 8 weeks). T17/524 xIMC 76 (13.60cm in 8 weeks) recorded the lowest height increment among all hybrids. The Nanays (2.87cm in 8 weeks), Parinaris (2.8cm in 8 weeks) and IMCs (2.59cm in 8 weeks) showed no significant differences in girth increments (P>0.05) but increase in girth for the Nanays and Parinaris differed significantly from increases in girth of Ts (2.36cm in 8 weeks). With a few exceptions among the Parinari clones some hybrid varieties, height and girth increments in seedlings exposed to capsids were significantly much lower than increments in seedlings that were not exposed to capsids (controls). Results from all the experiments conducted in the present study indicate that some of the Nanay hybrids (Na 744 x Pa 7/808, Na 440 x Pa 7/808, Na 279 x IMC 76, Na 260 x IMC 76), some Parinari hybrids (Pa 16 xNa33, Pa 150 x IMC 76, Pa 107 x IMC 76, Pa 7/808 x T16/613, Pa 184 x T16/613), and some T hybrids (T85/ 799 x Pa 7/808, T79/501 x IMC 76, T63/971 x IMC 76, T65/238 x Na 33, T79/467 x Pa7/808) and some Parinari clones (Pa 107, Pa 118 Pa 121 and Pa 7/808) are potential materials for the development of cocoa genotypes that are resistant/tolerant to capsid attack. Other clonal and hybrid materials such as T79/501, T79/467, T85/ 799, IMC 76, IMC 68 x IMC 49, IMC 11 x IMC 22, IMC 36 x IMC 47 and IMC 49 x IMC 68 which did not differ significantly in height and girth increments from the controls should be further investigated in future experiments to confirm their potential as breeding materials for capsid control.